Conventionally, flame sensors have been used in combustion devices such as boilers to detect whether or not a flame has been produced properly with in the furnace. The flame sensor detects whether or not there is a flame by detecting ultraviolet light that is outputted from the flame at the time of combustion.
In this type of flame sensor it is necessary to increase the luminous flux or the concentration of the incident ultraviolet light, in order to increase the sensitivity. While simply increasing the diameter of the flame sensor would increase the incident ultraviolet light, if such a large flame sensor were installed in the combustion equipment, it would cause too large a change in the shape of the furnace or in the capacity thereof, which could have a deleterious effect on the combustion reaction, or the like. Given this, conventionally a lens 102 that is transparent to infrared light has been disposed between the light source (the flame) and the flame sensor 101, as illustrated in FIG. 5, to focus the ultraviolet light that is incident on the lens 102, to thereby achieve an increase in the luminous flux or the concentration of the incident ultraviolet light onto the flame sensor 101 (See, for example, Japanese Unexamined Patent Application Publication 1107-113685).
However, because the material for tenses that are transparent to ultraviolet light (quartz glass, or the like) is expensive, mass production and cost reduction has been difficult.
Given this, the object of the present invention is to provide a flame sensor that can be manufactured at a relatively tow cost and that can increase sensitivity easily.